The invention relates to a method of acquiring images of a plane in a magnetic resonance apparatus, notably to a method of introducing a medical instrument into the body of a patient in a magnetic resonance apparatus where a position of a tissue to be treated has been determined in three dimensions and also a position of entry for the medical instrument, in which method a path to be followed by the medical instrument is defined by way of at least two different images of body cross-sections through said positions in the magnetic resonance apparatus, said medical instrument being guided along said path so as to reach the tissue to be treated, a plane to be imaged being defined by means of a further image in the magnetic resonance apparatus and the point of intersection of the path to be followed by the medical instrument and said plane to be imaged being indicated in said further image, the medical instrument being maintained at said point of intersection, viewed in the plane to be imaged, during its introduction.
A method of this kind is known from an article by S. G. Silverman c.s., Interactive MR-guided Biopsy in an Open-Configuration MR Imaging System; Radiology, Vol. 197, Oct. 1995, pp. 175-181. According to this known method the acquisition planes are orthogonal to one another, the plane to be imaged extending perpendicularly to the path to be followed by the needle. An optical tracking system which includes position measuring means in the form of LEDs (Light Emitting Diode) that co-operate with cameras, or in the form of one or more coils that are mounted on a medical instrument, for example a needle holder, is provided so as to ensure that the needle can be constantly maintained in the desired direction during its introduction into the relevant part of the body. In other words, the needle is controlled on the basis of the imaging planes that are rigidly defined relative to the magnetic resonance apparatus.
It is very important that the patient, or at least the relevant part of the body, is immobilized during the execution of the described method. For example, in the practice of treatment of tumors in the brain the head is immobilized in the magnetic resonance apparatus with the aid of clamping means. However, this constitutes a substantial burden to the patient. If the head were not immobilized in this example, the tumor could disappear from view in the imaging planes in the case of a change of position, that is, a displacement and/or change of orientation of the head; it would then be necessary to find the tumor again in an iterative manner, that is, by varying the imaging planes continuously until the center of the tumor is once more situated on the line of intersection of two relevant imaging planes. In many cases such treatment will become too long and too risky in those circumstances.
It is an object of the invention to avoid said drawbacks and to provide a method of acquiring images of a plane in a magnetic resonance apparatus which enables fast and accurate treatment without immobilizing the relevant part of the body.
To this end, the method in accordance with the invention is characterized in that the images are acquired in the magnetic resonance apparatus in imaging planes which are defined directly relative to reference points of a patient arranged in the magnetic resonance apparatus. Specific areas of the human anatomy can be used as such reference points, for example characteristic bone structures. It is also possible to fit artificial markers. Consequently, the reference system used is not formed by the magnetic resonance apparatus as is customary according to the present state of the art, but by a co-ordinate system which is linked directly to the patient per se, that is, without the intermediate step involving the co-ordinate system of the magnetic resonance apparatus. Such a method becomes possible notably when a position, that is to say a location and the orientation, of a relevant part of the body, for example the head of a patient, is defined and updated by means of position measuring means on a xe2x80x9creal-timexe2x80x9d basis, said position enabling the imaging planes to be defined time and again. The images situated in the imaging planes, therefore, are constantly adapted to the changing position and orientation of the relevant part of the body.
The invention relates not only to a method of acquiring images of a plane in a magnetic resonance apparatus, but also to a magnetic resonance apparatus which includes gradient coils. The magnetic resonance apparatus in accordance with the invention is provided with position measuring means for defining and updating a position of a part of the body of a patient on a xe2x80x9creal-timexe2x80x9d basis, and with control means which, in response to signals from said position measuring means, control the gradient coils in such a manner that images successively acquired in the magnetic resonance image are formed in imaging planes that are defined directly relative to reference points of the patient. A further embodiment of the magnetic resonance apparatus in accordance with the invention is characterized in that the position measuring means are formed by sensors to be attached to a relevant part of the body of a patient. A further embodiment of the invention is characterized in that the magnetic resonance apparatus is arranged to position a needle holder which is associated with a needle guide member, the position measuring means being formed by sensors fitted on the needle holder cum needle guide member which can be attached to the relevant part of the body of the patient so as to be centered relative to the entry position for the needle. With a view to the limited space available in a magnetic resonance apparatus, the sensors are preferably formed by coils for measuring the strength of the magnetic field at the area of the coils and by means for making corrections in a gradient field of the gradient coils associated with the magnetic resonance apparatus, said corrections being based on the variations of the strength of the magnetic field that are caused by changes in position of the coils. Evidently, use can also be made of other positioning means and sensors. The number of coils will be dependent on the number of degrees of freedom granted to the patient. In principle three coils are required for the head; more than three coils may be necessary for other, deformable parts of the body, whereas one coil may already suffice for a patient who is secured on a table that can be freely displaced in the longitudinal direction.